A well-known drawback of high power solid state lasers is that they suffer from thermo optic distortions that limit ultimately the amount of optical power that can be generated.
It is also well known that some problems with solid state lasers can be partially solved by the use of a gain medium having a liquid form as described in Patent Applications No US2003/0161364 by Michael D. Perry entitled “Laser Containing a Slurry” and No WO2007/095246 by Robert R. Rice entitled “Liquid Laser with Colloidal Suspension of Lasant Nanoparticles”.
Perry describes a liquid laser in which the gain medium comprises particles of a solid state material suspended within a fluid having a refractive index substantially similar to that of the particles. This slurry is circulated between a resonator and a cooling system, functioning therefore both as the gain medium and as a coolant. A drawback of this approach is that there must be a close match between the indices of refraction of the solid state particles and the liquid to avoid considerable Mie scattering losses.
Rice describes a liquid laser in which the gain medium comprises a liquid with nanoparticles of a solid state lasing material in a colloidal state. Due to the small dimension of the particles, losses incurred by Mie scattering of the laser light are less dependent on the indices' of refraction difference and generally negligible.
These liquid gain media are prone however to thermo optic distortions due, for example, to non-homogeneous pump-light distribution, large index of refraction dependency in temperature (dn/dT) or inhomogeneous flow. Accordingly, although the use of nanoparticles alleviates the index mismatch problem, there is still a need for improvement because of the abovementioned restrictions, as well as other issues associated with liquid gain media, such as quenching and the limited range of transparency of fluids.